1. Field of the Invention
The present invention relates generally to chalcogenide glass fibers, and more specifically, to the protection of chalcogenide glass fibers from the environment.
2. Background of the Invention
Currently there is great interest in IR transmitting glass fibers based on the chalcogen elements S, Se and Te. This is because, depending upon composition, these fibers transmit in the so-called "fingerprint" region between 2-12 .mu.m where practically all molecular species possess characteristic IR vibrational bands. Therefore, these fibers can be used in fiber optic chemical sensor systems using evanescent, absorption or diffuse reflectance types of probes for DOD facility clean up. In addition, these fibers are required for laser threat warning systems, IRCM and high energy IR (especially the 2-5 .mu.m region) laser power delivery systems to enhance aircraft survivability. In addition, these low-phonon energy chalcogenide glasses are excellent host materials for rare-earth doping. Fluorescence and laser transitions beyond 2 .mu.m are possible in these materials which might not be seen in other high energy phonon host materials such as silica and fluoride glass fibers due to multiphonon absorption. In addition, radiative emission at shorter wavelengths is enhanced. Doped chalcogenide glasses can be used in IR scene projection for IRCM systems as well as 1.3 .mu.m fiber amplifiers for telecommunications and fiber optic chemical sensor systems. In addition, numerous other applications exist for the systems.
For many practical applications, long-term mechanical strength is critical. While the chalcogenide glass fibers possess usable as-produce strengths, the long term durability and survivability of these fibers are problematic. Although chalcogenide glasses are chemically durable and do not exhibit a reaction with water, to some extent all fibers (including silica) undergo zero-stress aging and stress corrosion due to the presence of moisture. The water molecules attack the strained chemical bonds at the crack tips present on the fiber surface, causing a reduction in the fiber strength.
In addition, chalcogenide glasses are small band gap materials. It has been demonstrated that thin films of chalcogenide glass such as As.sub.2 S.sub.3 and As.sub.2 Se.sub.3 undergo both reversible and irreversible photo-structural changes in the presence of near band gap light, e.g., UV light and visible light. In the presence of air and moisture, the fiber surface becomes noticeably degraded with the subsequent catastrophic effect on the fiber mechanical strength. While UV light is above the band gap of these materials and therefore leads to enhanced degradation, researchers have observed similar degradation over a period of several months after exposing the fibers to ambient light in a laboratory. Therefore, there is a need to protect the chalcogenide fibers from UV, visible light, and moisture to prevent surface degradation and subsequent degradation of fiber strength.